1. Technical Field
This invention relates to image processing techniques and, more particularly, to improved techniques for matching patterns using area correlators.
2. Discussion
Area correlation is an important signal processing function in many image processors. Applications include trackers, pattern recognizers, scene stabilizers, and sensor line of sight alignment. Traditionally area correlation has been implemented using either greylevel product correlation or bilevel correlation. These two techniques represent opposite extremes in the performance/complexity tradeoff.
Generally, a product correlator performs the matching function by multiplying each pixel in a reference image with a corresponding pixel in a live image and accumulating the sum of the products. The demands of the sensor imaging rate, typically 60 Hz, often require parallel processing in hardware. The associated high cost, high power consumption and bulky hardware is unsuitable. In addition, the large amount of multiplications also limits the search range of the correlator.
The bilevel correlator performs a matching function by comparing the polarity of the spatial gradient between the reference image and the live image. A gradient operator generates the polarity of the spatial gradient for each pixel. The match function is computed by accumulating the number of pixel pairs in the two images with polarity coincidence.
The problem with conventional bilevel correlators is that for pixels in bland portions of the reference image, where the scene content is relatively uniform and the amplitude of the gradient is small, the polarity of the gradient often is dominated by noise. Statistically, the noisy pixels can add or detract from the correlation function with equal probability. Thus the inclusion of noisy pixels does not add to the mean peak of the correlation function but increases its variance, and reduces the signal to noise ratio at the output of the correlator. In bland scenes, the noisy pixels can overwhelm the number of signal-occupied pixels, causing the bilevel correlator to perform poorly.
An improvement in the performance of bilevel correlation is disclosed in pending U.S. patent application Ser. No. 229,407 filed on Aug. 8, 1988 and assigned to the assignees of the instant application. This pending U.S. application entitled "maskable Bilevel Correlator" retained the advantageous features of the bilevel correlator while alleviating the problems of noisy pixels. The automatic correlatable area selection technique selects areas having a high density of signal-occupied pixels for reference areas to enhance correlator performance in bland scenes.
To accomplish this, signals representing the live image and reference image are differentiated and thresholded to provide a polarity bit and mask bit for each pixel position. The value of the mask bit is an "off" digital state if the amplitude of the analog signal falls between two preselected threshold levels and is an "on" digital state if the amplitude is greater than the threshold levels. The value of the polarity bit depends upon whether the amplitude of the signal is positive or negative with respect to the level from which the thresholds are established. The polarity bits and mask bits of the resultant images generate a correlation function in which the polarity bit values are ignored for those pixel positions having mask bits in the "off" condition.